1. Field of the Invention
The present invention relates to elastomer-like or rubber-like, recyclable cross-linked polymers, methods for producing a molded article, and methods for recycling the polymers. Specifically, the present invention is to provide novel, elastomer-like or rubber-like, recyclable cross-linked polymers capable of being molded into articles of any desired shapes, which is characterized in that the wastes and the defectives of the molded articles as well as the molding wastes can be liquefied, while making the resulting liquids still maintain their intrinsic elastomer-like or rubber-like physical properties on a practicable level, and again re-molded into articles of any desired shapes.
2. Description of the Related Art
A technique is disclosed in U.S. Pat. No. 5,260,411 (prior art 1) and U.S. Pat. No. 4,882,399 (prior art 2) of introducing S--S bonds into the main chain of an epoxy resin or polyimide polymerized by addition (thermosetting), reducing the bonds (--S--S--.fwdarw.--SH HS--) to decompose (or liquefy) the resin into low-molecular moieties and, on the contrary, oxidizing the ends (--SH HS--.fwdarw.--S--S--) to re-cure the resin.
Ordinary, so-called vulcanized rubber (prior art 2) is produced by mixing raw rubber which is an aggregate of organic compounds with long carbon main chains, with sulfur or sulfur compounds. Because this process produces various sulfur cross-links such as --S-- bonds, --S--S-- bonds, --S--S--S-- bonds, etc. between the carbon main chains, the vulcanized rubber exhibits elastomer-like or rubber-like properties.
When the polymers of the above-mentioned prior art 1 and prior art 2 are desired to be recycled by liquefying the molded articles and following re-molding of the resulting liquids, there occur various problems such as those mentioned below.
Problems with prior art 1:
The proportion of S--S bonds to be introduced into a polymer, .rho. (number of S--S bonds in polymer:number of monomer units in polymer) is from 1:1 to 1:2 or so, and the density of the S--S bonds introduced into the main chain of the polymer is extremely high.
This is because epoxy resins and thermosetting polyimides need a high cross-link density in itself. Accordingly, S--S bonds must be introduced into them at high density so that melting of the cross-linked epoxy resins or polyimides should be relied only on the cleavage of the S--S bond itself that is introduced into their main chains. For these reasons, the high proportion of S--S bonds polymers is inevitable in the prior art 1.
This makes it difficult to liquefy the polymers within a short period of time by cleaving the S--S bonds thereof. This is because the penetration of an agent for cleaving S--S bond into the polymers is difficult because of high density of cross-links but rather a large amount of the agent must be introduced into the cross-linked polymers in order that the high-density S--S bonds in the polymers can be cleaved within a short period of time. For these reasons, the technique of the prior art 1 is not practicable in view of the processing speed.
Problems with prior art 2:
Since various sulfur cross-links, such as --S-- bonds, --S--S-- bonds, --S--S--S-- bonds, etc. are formed between the carbon main chains in the vulcanized rubber, it is difficult to effectively cut all these cross-links at the same time to liquefy the rubber. In particular, --S-- bonds are difficult to be cut and the vulcanized rubber with such --S-- bonds is not easy to be liquefied. If such --S-- bonds are forced to be cut under severe conditions, even the bond between carbons of the main chains are also cut. This raises another problem that the physical properties of units to be rebonded to a recycled rubber are changed.